The Ringworld is an artificial world with a surface area three million times larger than Earth's, built in the shape of a giant ring orbiting its sun, a million miles across and with a diameter of 186 million miles. It was built by the Pak, who later through infighting left it mostly Protector free. It is inhabited by a number of different evolved hominid species, as well as Bandersnatchi, Martians and Kzinti.

On Ringworld, time longer than a day is measured in falans, with 1 falan being 10 turns or 75 Ringworld days (93.75 Earth days), so 4 falans is slightly longer than 1 Earth year.

The "Ringworld" is an artificial ring about one million miles wide and approximately the diameter of Earth's orbit (which makes it about 600 million miles in circumference), encircling a Sol-type star. It rotates, providing an artificial gravity equivalent to 99.2% of Earth's gravity by way of Centrifugal force (fictitious). Ringworld has a habitable flat inner surface equivalent in area to approximately three million Earth-sized planets. The majority of the surface is land interspersed with shallow, freshwater seas. On opposite sides of the ring are two large deep saltwater oceans, placed in counterbalance to one another. One of the large oceans, known as the "Great Ocean", contains one-to-one maps of all of the inhabited worlds of known space. The "Other Ocean" has many maps of a single world: the Pak Homeworld. Walls 1000 miles tall along the edges retain the atmosphere. The Ringworld could be regarded as a thin, rotating slice of a Dyson sphere, with which it shares a number of characteristics. Niven himself thinks of the Ringworld as "an intermediate step between Dyson spheres and planets."

The Ringworld is described as having a mass approximately equal to the sum of all the planets in our solar system. The adventurers surmised that its construction consumed literally all the planets in that system, down to the last asteroid and/or moon, as the Ringworld star has no other bodies in orbit. In Ringworld's Children it is additionally explained that it took the reaction mass of roughly 20 Jupiter masses to spin up the ring; thus the combined mass of the planets of the original system was that much larger than our solar system's.

Scrith, usually written italicized as scrith, forms the walls and floor of the Ringworld.

Scrith is a milky-gray translucent, nearly frictionless material. The fairly thin layer of scrith that forms the floor of the Ringworld blocks the passage of 40% of the neutrinos that encounter it, equivalent to almost a light year of lead. It also absorbs nearly 100% of all other radiation and subatomic particles and rapidly dissipates heat. The tensile strength of scrith is similar to the strong nuclear force, with the Ringworld foundation only about 30m (100 ft) deep. Also, it is transparent to large magnetic fields.

Due to its enormous strength, scrith is impervious to most weapons. A body (such as a comet or asteroid) striking with enough kinetic energy may be able to deform the Ringworld floor and punch a hole. The Ringworld engineers used a device, called the cziltang brone in their language, to pass from the vacuum of their spaceports right through the scrith to the habitable surface of the Ringworld.

The physical composition of scrith is unclear, but it appears to share some of the properties of a metal (albeit in a greatly exaggerated form): for instance, the high tensile strength, the ability to conduct heat and the ability to retain an induced magnetic field. Scrith is said to have been artificially produced through the transmutation of matter.

Recently in Destroyer of Worlds (novel) it was revealed that by combining aspects of the Pak building material twing, itself an incredibly durable programmable substance, with the theory of resonant molecule bonds could produce something akin to Scrith. Most notably as both Scrith and Twing can be reshaped to pass through thanks to a softening deivce (Pak) or a cziltang brone per the Ringworld.

The construction of a ringworld remains firmly in the area of speculation. If such a structure were built it could indeed provide a huge habitable inner surface, but the energy required to construct it and set it rotating is so significant (several centuries' worth of the total energy output from the Sun) that without as-yet unimagined energy sources becoming available, it is hard to see how this construction could ever be possible in a time frame acceptable to humans.

The tensile strength of the material required would be on the same order as the strong nuclear force, according to Niven — since the artificial gravity is the same as normal gravity, the structure is comparable with a bridge with an extremely long span; nothing even remotely strong enough is known to exist in nature. In Niven's Ringworld novels, the material—which he calls scrith—is said to have been artificially produced through the transmutation of matter into the required substance. (This merely gives a name to the sufficiently advanced technology that would have to be used.) In later novels the "transmutation" idea is simply discarded and the construction method of scrith left open, although one engineer (Tunesmith) is able to use nanotechnology to weave new scrith into meteor punctures.

A ringworld design requires active stabilization, because it is not in inertialorbit. Though the ring itself is rotating at 1200 km/s (to approximate Earth gravity), the center of mass is stationary — in fact, it is at an unstable equilibrium, roughly comparable to a small sphere balanced on top of a larger one.

Thus, large thrusters must be incorporated into the design to keep it centered about its star. This point gave Niven some difficulty after he published his first Ringworld novel; he was deluged with letters pointing out that "the Ringworld isn't stable" and dedicated the first sequel to a resolution of this problem. He notes in the dedication of Ringworld Engineers that at the 1971 World Science Fiction ConventionMIT students crowded the hotel hallways chanting "The Ringworld is Unstable!" In this first sequel, he also tackled how to prevent all the soil from ending up in the oceans. In the fourth book in the series, Ringworld's Children, he creates backplot explanations for several of the imperfections in his original design of the Ringworld — and wholly glosses over others, such as that Louis Wu is worried about his dietary intake of salt since only the Great Oceans are described as being saline.

To provide an approximation of the day–night cycle common to planets, Ringworld was also provided with a separate ring of "shadow squares" linked together (by "shadow square wires") in a ring close to the star, rotating at slightly faster than the Ringworld's spin, providing a lot of twilight, as well as a day-night cycle. This is not the perfect match for a planet however, as there is no sunrise or sunset in Ringworld, and when not covered by a shadow square, the sun is always at high noon. These absorb a huge amount of sunlight energy, which is beamed to the Ringworld as its primary source of power. They are also not in inertial orbit, and must be actively stabilized as well. The shadow squares provide another of the imperfections "clarified" in Ringworld's Children, as five shadow squares of greater length, orbiting retrograde would provide a better day-night cycle, with less twilight. As revealed in Ringworld Engineers, the "shadow squares" also provide a shielding to the inner surface of the Ringworld when someone in the control room uses a magnetic field embedded in the Ringworld to fire the meteor defense system.